This invention relates to an improvement in the cold trap of a type having a heat-insulating gas layer between a cooling section which cools an inflowing liquid sodium and a trapping section which traps impurities in the liquid sodium.
A cold trap is a device for refining liquid sodium by precipitating impurities in the sodium due to the differences in saturated solubility (when liquid sodium containing impurities is cooled to about 140.degree. C., the impurities become supersaturated and are separated out) and by trapping the thus precipitated impurities in meshes disposed in an impurity trapping section of low temperature. In conventional cold traps, as typically illustrated in FIG. 1, a heat-insulating gas layer 3 is provided between an inflowing liquid sodium cooling section 1 and an impurity trapping section 2 so as to prevent the exchange of heat between the impurity trapping section (low-temperature section) 2, and the sodium cooling section 1 which is higher in temperature than the trapping section. Thus, the impurity trapping section 2 in which the meshes are filled is made uniform in temperature, thereby allowing the effective utilization of the entire volume of meshes for trapping the impurities. Reference number 13 indicates a heat-conducting pipe adapted for passing a cooling gas which cools the inflowing liquid sodium in the cooling section 1. Reference numbers 14 and 15 indicate a sodium inlet pipe and a sodium outlet pipe, respectively.
The heat-insulating gas layer 3 comprises a double-walled structure consisting of an inner cylinder and an outer cylinder which are closed at both upper and lower ends. An inert gas such as argon or nitrogen is enclosed in the space in this double-walled cylindrical structure. This space communicates with a gas reservoir 5 at the top of the cold trap through a pressure equalizing pipe 4 so as to eliminate any pressure difference between the heat-insulating gas and the sodium to prevent the likelihood of excess stress in the sides of the double-walled structure.
However, with cold traps of such a conventional construction, if a large temperature difference is produced between the inner and outer cylinders of the heat-insulating gas layer 3, the component members of the layer may be deformed or damaged because of the differences in the amount of thermal expansion or contraction between the inner and outer cylinders. In the worst case, the heat-insulating gas could escape, allowing sodium to take its place, thus spoiling the heat-insulating effect of the gas layer and making it incapable of keeping the impurity trapping section 2 at a uniform temperature, resulting in the badly impaired function of the cold trap. Such a phenomenon occurs because of the closing bottom plate 6 provided at the lower end of the double-walled cylindrical structure forming the heat-insulating gas layer in such conventional cold trap, but such a bottom plate 6 is indispensable for the proper functioning of the cold trap.
Under these circumstances, the conventional cold trap are subject to a serious operational restriction, that is, the system must be operated with care to ensure that the temperature difference between the inner and outer cylinders of the heat-insulating gas layer do not exceed a predetermined level (usually about 60.degree. C.) when the temperature of the system is elevated. Thus, for instance, in a purification system of the general testing equipment of a reactor mock-up, it is impossible to raise the sodium temperature in the system above 350.degree. C. in normal operations even if an economizer is provided on the cold trap inlet side. Also, when re-starting after a temporary shutdown of the system due to a power failure or other causes, it is necessary, before re-starting to circulate the sodium, to lower the temperature of the sodium in the system to close to the cold trap temperature so as not to give rise to a temperature difference greater than the level predetermined by the heat-insulating gas layer.